One more time, wireless doesn't mean "no wires." This may seem illogical, but wireless in most process applications usually needs added cabling to provide power to new transmitting and receiving devices. Those transmitters need wires to secure signals from sensors and I/O points before going wireless, while those receivers need wires to send their data via fieldbuses, Ethernet, serial and other conductor-based networks to process controls, servers and enterprise systems.
However, even though wireless doesn't stay wireless for long, the fact that it can go airborne has opened up a world of new process control capabilities that most engineers, integrators and end users have only started to mentally catch up with in the past couple of years.
Rocks and Hard Places
In the beginning, everyone focused on the cabling and connectors wireless could save by simply replacing existing hard-wired links. But many are gradually realizing that wireless also allows them to deploy sensors and instruments in places they could never go before due to too little space or too much expense, and bring back more signals and better information that was formerly out of reach.
For instance, SSE's 80-MW, combined heat and power (CHP) plant in Slough, U.K., used to rely on manual sampling and laboratory analysis of its turbine condensate, but this method caused delays in detecting leaks, so its boilers had unplanned shutdowns for repairs. It's crucial to detect changes in boiler condensate conductivity because they can be caused by cooling-water leaks in the turbine's condenser. If left undetected, contaminated feed water can cause hydrogen embrittlement of boiler furnace tubes and cause them to fail.
Slough Heat and Power's boiler house previously used a Smart Wireless Starter Kit from Emerson Process Management, so SSE decided to deploy its Rosemount Analytical Model 6081C wireless conductivity transmitters in the boilers' condensate extraction lines (Figure 1). Emerson also recommended best positioning for the conductivity probes, transmitters and gateway.
"These wireless conductivity transmitters allow us to continuously monitor the condensate extract lines," says Emma Wilcockson, SSE's electrical, control and instrumentation technician. "If we detect a change in conductivity, maintenance can be scheduled before the problem leads to an unplanned shutdown or damage."
The conductivity transmitters send data to the plant's PLC-based controls system via the gateway, and then Emerson's AMS Device Manager predictive maintenance software manages the wireless network. Wilcockson reports it was easy for SSE's engineers to install and configure the new plug-and-play wireless conductivity devices in the plant's turbine basements. "Despite the harsh environment of the turbine basement, which is surrounded by metal structures that can obstruct wireless signals, our wireless network was quickly established," adds Wilcockson. "The system has been operating for more than six months, and during that time the transmissions between transmitters and the wireless gateway have been extremely reliable."
Likewise, process applications from steam traps to injection wells can make similar gains in maintenance and performance by adding wireless devices, according to Bob Karschnia, Emerson's vice president of wireless. "Steam traps remove water from plant processes, but they degrade over three or four years and can leak and get stuck open. However, by using wireless transmitters to look at them regularly, some users can save $4 million per year in the gas they use to make steam.
"We also have customers drilling wells and injecting steam to free oil, and they're using wireless on their wellheads instead of wired, which can reduce installation time from about 36 hours to just eight hours. Reducing start time by an entire day means oil can flow a day earlier, which can pay for the whole wireless setup. So wireless isn't just about saving on hardware; it's about gaining more intelligence and the value of starting up sooner. Some users are putting in applications where wireless is 40% of their total I/O count."
Plays Well with Others
Because wireless needs cabling on either end to transmit from sensors and I/O points and to receive data for PLCs and upper level systems, it's crucial for wireless components to interact and cooperate well with these wired systems.